About DSL Internet Service Providers

Twisted Pair Access to the Information Highway

Digital Subscriber Line (DSL), a new modem technology, converts existing twisted-pair telephone lines into access paths for multimedia and high speed data communications. DSL can transmit more than 6 Mbps downstream (to a subscriber) and 640 Kbps upstream, and as much as 1.1 Mbps in both directions. Such rates expand existing access capacity by a factor of 50 or more without new cabling. DSL can literally transform the existing public information network from one limited to voice, text and low resolution graphics to a powerful, ubiquitous system capable of bringing multimedia, including full motion video, to everyone's home this century. DSL will play a crucial role over the next ten or more years as tele- phone companies enter new markets for delivering information in video and multimedia formats. New broadband cabling will take decades to reach all prospective subscribers. But success of these new services will depend upon reaching as many subscribers as possible during the first few years. By bringing movies, television, video catalogs, remote CD-ROMs, corporate LANs, and the Internet into homes and small businesses, DSL will make these markets viable, and profitable, for telephone companies and application suppliers alike.

Capabilities

DSL modems provide data rates consistent with North American and European digital hierarchies (see Table 1) and can be purchased with various speed ranges and capabilities. The minimum configuration provides 1.5 or 2.0 Mbps downstream and a 16 kbps duplex channel; others provide rates of 6.1 Mbps and 64 kbps duplex. Products with downstream rates up to 8 Mbps and duplex rates up to 640 kbps are available today. DSL modems will accommodate ATM transport with variable rates and compensation for ATM overhead, as well as IP protocols. Downstream data rates depend on a number of factors, including the length of the copper line, its wire gauge, presence of bridged taps, and cross-coupled interference. Line attenuation increases with line length and frequency, and decreases as wire diameter increases. Ignoring bridged taps, DSL will perform as follows:

Data Rate	Wire Gauge	Distance	Wire Size	Distance
1.5 or 2 Mbps	24 AWG	18,000 ft	0.5 mm	5.5 km
1.5 or 2 Mbps	26 AWG	15,000 ft	0.4 mm	4.6 km
6.1 Mbps	24 AWG	12,000 ft	0.5 mm	3.7 km
6.1 Mbps	26 AWG	9,000 ft	0.4 mm	2.7 km

While the measure varies from telco to telco, these capabilities can cover up to 95% of a loop plant depending on the desired data rate. Customers beyond these distances can be reached with fiber-based digital loop carrier systems. As these DLC systems become commercially available, telephone companies can offer virtually ubiquitous access in a relatively short time.

Many applications envisioned for DSL involve digital compressed video. As a real time signal, digital video cannot use link or network level error control procedures commonly found in data communications systems. DSL modems therefore incorporate forward error correction that dramatically reduces errors caused by impulse noise. Error correction on a symbol by symbol basis also reduces errors caused by continuous noise coupled into a line.

Technology

DSL depends upon advanced digital signal processing and creative algorithms to squeeze so much information through twisted-pair telephone lines. In addition, many advances have been required in transformers, analog filters, and A/D converters. Long telephone lines may attenuate signals at one megahertz (the outer edge of the band used by DSL) by as much as 90 dB, forcing analog sections of DSL modems to work very hard to realize large dynamic ranges, separate channels, and maintain low noise figures. On the outside, DSL looks simple -- transparent synchronous data pipes at various data rates over ordinary telephone lines. On the inside, where all the transistors work, there is a miracle of modern technology.

To create multiple channels, DSL modems divide the available bandwidth of a telephone line in one of two ways -- Frequency Division Multiplexing (FDM) or Echo Cancellation. FDM assigns one band for upstream data and another band for downstream data. The downstream path is then divided by time division multiplexing into one or more high-speed channels and one or more low speed channels. The upstream path is also multiplexed into corresponding low speed channels. Echo Cancellation assigns the upstream band to over-lap the down- stream, and separates the two by means of local echo cancellation, a technique well know in V.32 and V.34 modems. With either technique, DSL splits off a 4 kHz region for POTS at the DC end of the band.

A DSL modem organizes the aggregate data stream created by multiplexing downstream channels, duplex channels, and maintenance channels together into blocks, and attaches an error correction code to each block. The receiver then corrects errors that occur during transmission up to the limits implied by the code and the block length. The unit may, at the user's option, also create super- blocks by interweaving data within sub-blocks; this allows the receiver to correct any combination of errors within a specific span of bits. This allows for effective transmission of both data and video signals alike.

Standards and Associations

The American National Standards Institute (ANSI), working group T1E1.4, recently approved a DSL standard at rates up to 6.1 Mbps (ANSI Standard T1.413). The European Technical Standards Institute (ETSI) contributed an Annex to T1.413 to reflect European requirements. T1.413 currently embodies a single terminal inter- face at the premise end. Issue II, now under study by T1E1.4, will expand the standard to include a multiplexed interface at the premise end, protocols for configuration and network management, and other improvements.

The ATM Forum and DAVIC have both recognized DSL as a physical layer transmission protocol for unshielded twisted pair media.

The ADSL Forum was formed in December of 1994 to promote the DSL concept and facilitate development of DSL system architectures, protocols, and interfaces for major DSL applications. The Forum has more than 300 members representing service providers, equipment manufacturers, and semiconductor companies from throughout the world.

Market Status

DSL is currently undergoing mass deployment in North America and parts of Europe. Several telephone companies and Local Exchange Carriers have entered the DSL market and are aggressively installing DSL equipment in order to make it commercially available in a growing number of local markets.

Semiconductor companies have introduced transceiver chipsets that are currently being used in numerous markets. These chipsets combine off the shelf components, programmable digital signal processors and custom ASICS. Continued investment by these semiconductor companies has increased functionality and reduced chip count, power consumption, and cost, enabling mass deployment of DSL-based services.

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